In some conventional imagers, transparent images are sequentially scanned by projecting light through the image and onto a CCD imaging array. The signal from the CCD array is conditioned so that a video output is generated from scanned image. The images may consist of typical 35 mm slides or other forms of photographic negatives. The CCD array typically consist of photo diodes disposed in rows and columns and layed out to form an array conforming to a standard video format, such as the NTSC standard or the PAL standard, for example. The photo diodes may be covered with a color filter array of various patterns that generate color as well as luminance data. The array responds to the light from an internal light source that is projected through the image-bearing transparency and focused onto the imager. Currently, thermal printers are designed to capture the resulting video signals in a digital memory and make a thermal print. Conventional thermal printers employ analog capture circuitry responsive to the video signals and sufficient memory to store the complete signal digitally. The thermal printer then generates thermal hard copy by sequential deposition of dye donor onto a dye receiver in typical fashion.
Thermal prints are desirable, especially color prints. One weakness of the conventional methods used to generate video display and create thermal prints is that the resolution is not the best available. Typically, a 35 mm slide or negative has several times the resolution of the imager which is typically about 500.times.500 pixels. The resolution is optimized for video output but below the capabilities of thermal printing. A typical thermal printer in such applications uses a 6 dot per millimeter head to produce a 3.times.4 inch image on a sheet of receiver. Thermal printing technology does exist to double the pitch of the head to 12 dots per millimeter which will provide a significant improvement in image quality. Unfortunately, the means of printing an electronically generated image from high resolution silver halide to higher resolution thermal printing is limited by the resolution of the CCD which is in turn defined by video resolution. Accordingly, it will be appreciated that it would be highly desirable to generate a thermal print from a high resolution silver halide image.
Even though the silver halide image has extremely high resolution, the thermal print generated is limited by the resolution of the CCD display which is on the order of 500.times.500 pixels or so, whereas the silver halide image has a resolution which is several times greater. Imaging is performed by passing light through the transparent image onto the CCD and storing the image as a frame of data for subsequent printing by the thermal printer. What is actually on the transparent image may not be exactly what is seen by the CCD since the CCD is only seeing things up to the ability of its resolution. To get a more accurate reproduction and thereby greater resolution, the CCD has to see more.
One method of having the sensor see more is disclosed in U.S. Pat. No. 4,638,371, for Multiple Exposure of Area Image Sensor Having A Sparse Array of Elements which issued Jan. 20, 1987 to James R. Milch. Pixels of a digital image are produced by an area image sensor which includes a sparse array of elements. Each element is multiply exposed by different pixels of a light image. The light image is scanned in such a pattern between element exposures that each digital image pixel has a nearest neighbor digital image pixel that was produced by a different sensor element. By this arrangement, a high quality image can be produced from the digital image even if a sensor becomes defective. A dithered sensor is disclosed that has a plurality of positions so as to create a sub-image. The system disclosed, however, does not show an embodiment consisting of a single sensor suitable for video output. It is therefore desirable to have a dithering system using a single sensor to provide a video output.
U.S. Pat. No. 4,668,978, for Thermal Transfer Color Image Forming Apparatus With Image Color and Image Color Density Control Functions which issued May 26, 1987 to Masami Gokita, discloses a color and density mode control system for an image forming apparatus including and optical scanner and a color specifying unit for specifying the mode or colors that the apparatus is to use in making copies. Depending on the color mode selected, colors with or without halftones are used in forming copies. A color converter is also used for converting image information obtained by the optical scanner into color signals as specified by the color specifying unit. While a scanning copier is essentially described, a dithering process is used; however, the sensor is not moved. Dithering is the use of an array of pixels that are color (i/o) on or off that are selectively energized so as to create a macro pixel with greater color depth. In addition, the scanning element is a linear array that is mechanically moved in a direction perpendicular to the array of scanning elements. The image capture mechanism does not have a video output capability.
Solid state image sensors generally have a linear or area organization. An area image sensor offers the advantage of increased integration time for each element. In some applications, a large number of image pixels have to be digitized. For example, to make a high quality color print of a photographic negative, something on the order of about two million image pixels should be digitized for each color (red, green and blue) of a photographic negative to produce a high quality output print. With existing technology, typical consumer area image sensors have about 300,000 elements. Thus, each element of an area image sensor must sample a plurality of image pixels.
It is apparent that dithering is a technique to create an image of higher resolution than the sensor's normal capability. Accordingly, it will be appreciated that it would be highly desirable to use the dithering concept to obtain a high resolution thermal image from a video imager using a transparency.